I've heard they've made many improvements on it in the last couple of years, but if it's in the same condition it was when I did it (sometime between '94 and '97 or so), then yeah, don't take your motorhome on it. We did it going 35mph most of the way in a Toyota Tercel and had to replace the shock absorbers upon our return to Anchorage (the drive back up the Glenn felt like we were in a speedboat jumping wave crests...).

Also, as Quokka said, you can't actually drive to McCarthy or the Kennicott mine itself. You park in a large lot on the banks of the Kennicott River and walk on a pedestrian-only bridge. After crossing the river, you can either walk the half a mile or so to the town and the five or so miles on to the mine ruins or there are vans that run around the area for a fee. (Google Maps satellite imagery of the pedestrian crossing here and of the road to the Kennicott mine here.) In 1993 or so, my aunt went and told tales of having fun hauling herself across the river in a hand-pulled tram, but by the time we got there, they'd built the pedesterian bridge. (There's still a fun hand-pulled tram at Wonder Creek near the Alyeska hotel in Girdwood, though.)

If you can manage to go, it is a beautiful area (picture of the mine here, with the Wrangell-St. Elias mountains in the background), and apparently there are now tours of at least something (it was disappointing for me when I went because there were no tours--we just poked around the outside of the buildings and then left). But it's a long and harrowing drive to get there...